Lasers are resonators which contain an amplifying medium that functions to increase the intensity of the light that passes through it. The amplifying medium may be a solid, liquid or a gas. For example, in a neodymium YAG (Nd:YAG) laser, the amplifying medium is a solid rod of yttrium aluminum gamate (YAG) containing neodymium ions. Another example of a solid state laser is a laser diode. In a laser diode, also known as a diode laser or semiconductor laser, a semiconductor junction is sandwiched between a p-type semiconductor layer and an n-type layer semiconductor layer. Laser diodes are generally electrically pumped by passing an electric current across the p-n junction.
Without locking, certain lasers provide too wide a spontaneous emission spectrum line width for some applications or they provide emission spectrum lines that hop from one wavelength to another in response to small changes in environmental conditions, such as temperature. The diode laser is one such laser. The result is a need for wavelength locking filters for applications which require precise control of the laser output. These filters have high transmittance at the desired wavelength and very steep cut-off at adjacent wavelengths.
Thus, wavelength locking of light emitting semiconductor devices provides selection of only light of a specific wavelength (generally the fundamental wavelength) from within the spontaneous emission spectrum of a device to be injected or reflected back into the gain region of the device, thereby “locking” the laser output. Wavelength locking can be implemented using external configurations, such as an external cavity including a Fabry-Perot filter, or using internal (integrated) configurations. As used herein, the term “integrated” refers to a one piece article beginning with a bulk substrate material (such as a Si or GaAs wafer) which includes deposited and doped regions, lithographically defined regions, etched regions and etched and filled regions, formed using integrated circuit processing techniques. Integral members may be contrasted with devices or articles which include a plurality of separate and non-integrated components.
One common example of integrated wavelength locking of semiconductor lasers is using distributed feedback lasers (DFB), in which a Bragg grating is fabricated over the entire length of the gain section. Another common example is the distributed Bragg reflector (DBR), in which a pair of Bragg gratings is fabricated in a passive region on both sides of the gain section of the device to provide optical feedback.